With the advent of new, potentially large sales volume applications of superconductive magnets, there is renewed interest in the physics and the economics of the manufacture of superconductive wire and superconductive coils. For example, it is anticipated that a substantial portion of the high field strength nuclear magnetic resonance imaging systems will employ superconductive magnets. Perhaps, hundreds of these magnets will be manufactured per year, transported to hospital and clinical sites, and undergo virtually continual hour-to-hour and day-to-day use thereafter. It is most desirable, then, that these magnets, which are essentially consumer commodities, should be of the utmost stability and should involve persistence characteristics of the highest order. In particular, it is important to consider that each time a superconductive magnet quenches (i.e. vaporizes and exhausts cryogenic liquids due to the heat generated as the magnet becomes resistive), the entire imaging system must go offline to re-establish uniform cryogenic conditions and to re-establish a persistent superconducting state. This situation is an inconvenience in any event, but will be all the more critical if a multimillion dollar clinical imaging system is forced offline during the process.
It is, therefore, a general object of the present invention to provide superconductive wire designs and configurations which substantially improve the persistence characteristics thereof.
Superconductive wire joints provide one common source of resistivity in superconductive coils. For small coils, consisting of a superconductive filament and copper cladding, wire-to-wire joints are easily made and good persistence is achieved. As the coils become larger, for example those such as are utilized in nuclear magnetic resonance imaging systems, it is desirable to utilize superconducting wire with many superconducting filaments. See, for example, the wire designs set forth in U.S. Pat. No. 3,625,662 to Roberts et al., entitled "SUPERCONDUCTOR". That patent teaches, for example, the use of multiple superconducting filaments embedded in a matrix of nonsuperconducting metal, such as copper. Establishing wire-to-wire joints for such multifilamentary wire is, however, a formidable task. Even the most careful and exacting procedures for establishing a joint result in some amount of poor registry and/or poor electrical coupling between associated elements across the joint. In the end, there is effectively established a resistance matrix between each superconducting filament on one side of the joint, and each of the others on the other side of the joint. This establishes a semi-unique current distribution across each joint, depending upon the structure giving rise to the joint. General objects in the art of multifilamentary wire joints, therefore, are to make all these interconductor resistances quite small. In fact, however, the prior art is understood to provide little in the way of insuring, a priori, that a complete, very low resistance joint will be established. In other words, conventional wisdom has been that one must be content to live with some small amount of resistive transfer at the joint, together with the concomitant uncertainty as to how significant that resistance will be.
It is an object of the present invention to provide multifilament superconducting wire designs and configurations which eliminate or at least minimize with certainty the degree of resistive energy transfer which will occur at wire joints. It is a related but by no means insignificant object that such designs be relatively compatible with prevailing joint making techniques, and that the resulting wire involve material selection and cost constraints which are comparable to those of conventional multifilamentary wire.